This invention relates to improved ultrasonic soldering of drawn aluminum tube without flux. More particularly, this invention relates to a method for producing aluminum tube which is conditioned so as to be more readily soldered by ultrasonic fluxless soldering techniques.
Ultrasonic fluxless soldering of aluminum tube joints in the manufacture of heat exchangers is well known in the art as shown by "Ultrasonic Soldering Gives Aluminum a Boost", Modern Metals, August 1974, pp. 97, 100 and 102, and U.S. Pat. Nos. 3,760,481; 3,822,820; 3,831,263; 3,084,650; 3,920,176 and 3,833,986, all of which are incorporated herein by reference. In these processes a multiplicity of joints is assembled to provide a heat exchanger, and the joints are soldered by immersing in a pot of molten solder alloy without the use of a chemical flux. The joints are typically socket-tube joints wherein a tube is inserted into a socket provided by expanding and outwardly flaring the end portion of a tube of like size or diameter. Ultrasonic excitation of the bath facilitates wetting of the aluminum tube members which allows for eliminating the chemical flux and the attendant cleanup problems. It is important in ultrasonic soldering that the filler metal wet and adhere to the parent tube members. However, with certain aluminum tube products, difficulties have been experienced in making the joints in that sometimes the solder alloy did not properly wet the aluminum tubing. The problem in many instances is more prominent where the aluminum tubing is made by continuous drawing techniques as opposed to extrusion.
Extruded aluminum tube products are produced by hot extrusion, for instance at temperatures of 800.degree. to 1000.degree. F., essentially to its final tube dimensions in a single step starting with a billet of about 6 inches in diameter and finishing with a tube about 3/8 inch in diameter. In some cases, extruded tube is drawn through a sink die which reduces its cross section by about 0.01 to 0.015 inch in order to improve dimensional tolerance control and provide a slight increase in strength over the essentially annealed character of the tube as it exits the extrusion operation. Extruded tubes can usually be ultrasonically soldered without difficulty irrespective of whether the sinking operation is used. The internal structure of hot extruded tube is recognized as typically characterized by a mixture of unrecrystallized hot worked structure and coarse recrystallized structure.
The starting stock for the continuous drawing operation is typically a hollow extrusion about 21/4 inches in diameter made by hot extruding an 18-inch diameter billet. The extrusion is then drawn through a sequence of constricting dies of diminishing diameter, typically about 10 such dies in sequence. Each such draw operation employs a floating bulb mandrel and reduces the outside and inside diameters and the wall thickness of the aluminum stock and the sequence produces a coilable tube product about 1/4 to 1/2 or 3/4 inch, typically 3/8 inch, in diameter which is typically annealed to soften the work hardening effects of the draw operations. The internal structure of such drawn tube is recognized as typically characterized by a fine grained condition because of the cold working and annealing. Aluminum tubing produced by the aforesaid continuous sequential drawing sometimes experiences more rejections than extruded tube when made into heat exchanger assemblies by ultrasonic soldering because of "leaker" joints caused by less wetting of the tube joining surfaces by the solder alloy. While, in some cases, this may be attributed to improper cleaning or other solder process variations, it nonetheless appears that continuously drawn aluminum tube is more sensitive to variations in the process including the preliminary cleaning steps than extruded tube products. The sensitivity of the continuously drawn tube is not reduced any by well known and respected preconditioning methods, such as abrading with steel wool or sandpaper. This sometimes only aggrevates the problem and further reduces the tube wetting during soldering.